Ongoing efforts are being made in the field of microelectronic devices to achieve higher circuit density. One method of increasing the number of components per chip is to decrease the minimum feature size on the chip, which requires higher lithographic resolution. This has been accomplished over the years by reducing the wavelength of the imaging radiation from the visible (436 nm) down to the ultraviolet (365 nm) and to the deep ultraviolet (DUV) at 248 nm. More recent efforts at developing commercial lithographic processes use ultra-deep ultraviolet radiation, particularly 193 nm and 157 nm. See, for example, Allen et al. (1995) J. Photopolym. Sci. and Tech. 8(4):623, and Abe et al. (1995) J. Photopolym. Sci. and Tech. 8(4):637.
Despite these developments, there are no negative resists that are commercially available for 193 nm and 157 nm lithography. A recent study suggests that negative resists may have a better process window than positive resists for printing narrow trench geometries. See Brunner et al. (2001), Proc. SPIE 4345, 30. Therefore, there is a need to develop negative resists for 193 nm and 157 nm lithography.
Commercial resist candidates for use in 248 nm lithography are all based on cross-linking mechanisms. The problem mostly encountered with cross-linking systems is the micro-bridging of the features, where tiny strings of undeveloped resist material are left in the space between resist lines. Unfortunately, micro-bridging would become more pronounced in 193 nm and 157 nm lithography because of the smaller features.
Another approach, the acid catalyzed polarity switch (from polar to non polar) for negative resists, has been proposed, but has never resulted in a commercial resist. The polarity switch mechanisms involve pinacol rearrangement (from diol to carbonyl) or dehydration of tertiary alcohols attached to styrene based polymers followed by the cross-linking of the vinyl styrene. See Ito et al. (1994) Polymeric Materials for Microelectronic Applications, ACS Symposium Series 579, American Chemical Society, Washington, D.C., and references cited therein. Since these mechanisms are generally based on aromatic polymers (e.g., styrene based polymers), they are generally unsuitable for 193 nm and 157 nm lithography because aromatic polymers are opaque at 193 nm and 157 nm. These mechanisms also do not work well with standard developers. Recently, however, attempts have been made to apply a dehydration mechanism in cycloolefin polymers in order to develop 193 nm negative resists. See Fu et al. (2001) Proc. SPIE 4345, 751. However, an organic developer was needed to develop the resist.
Researchers have also attempted to use acid-catalyzed lactone formation of hydroxy acids for 193 nm negative resist development. See Yokoyama et al. (2001) Proc. SPIE 4345, 58, and references cited therein. This type of system normally requires a very weak developer, the product of which has poor shelf-life since the hydroxy acid slowly shifts back into the lactone at room temperature.
Therefore, there remains a need in the art to develop negative resists for 193 nm and 157 nm lithography. This invention addresses that need.